The objectives of this project are to use molecular and classical approaches to investigate pathogen-arthropod interactions of vector-borne agents causing diseases of human importance in the United States. Most of our effort has concentrated on Yersinia pestis, the causative agent of bubonic plague, Borrelia hermsii, an agent of tick-borne relapsing fever, and spotted fever group rickettsiae. We examined the role of the hmsHFR locus of Y. pestis in infection and subsequent blockage of fleas. The hmsHFR locus is essential for hemin storage (Hmsa+) in the outer membrane and formation of pigmented colonies on hemin and Congo red agars at 26-30 degrees C. Oriental rat fleas (Xenopsylla cheopis) were fed blood containing 5x10/8 bacteria per ml of one of 6 strains of Y. pestis, using an artificial feeding device. Fleas fed a blood meal infected with Hms+ stains exhibited high mortality (41- 63% after 4 weeks) and blockage of the midgut (37-42%). Fleas fed uninfected blood, or blood infected with either of 2 Hms- mutants showed normal mortality (5-13%) and no blockage. High mortality and blockage were restored when an Hms- strain was transformed with a plasmid containing the intact hms locus. These data suggest that hemin storage in the outer membrane is essential for Y. pestis to block the midgut of the rat flea, which is a critical event leading to efficient transmission of plague. Tick-borne relapsing fever of humans in the western United States is presently known to be caused by two or three closely related species of Borrelia spirochetes transmitted by Argasid ticks in the genus Ornithodoros. How these spirochetes vary antigenically in their tick vector, and how different serotypes of the spirochete affects tick infection and transmission, are unknown. During the last year we infected two cohorts of nymphal Ornithodoros hermsi ticks with different serotypes of B. hermsii. The first group of ticks was infected with serotype 7, and the second group of ticks was infected with serotype 8. The ticks were held at 27 degress C until they had molted to the third nymphal stage and were again ready to feed. Single ticks from each group were allowed to feed on individual mice and then the peripheral blood of the mice was examined daily for 14 days to determine if ticks had transmitted spirochetes. A total of 95 ticks was fed on individual mice: 16 of 46 ticks infected with serotype 7 transmitted spirochetes (35%) while only 2 of 49 ticks infected with serotype 8 transmitted spirochetes (4%). This is the first demonstration that the frequency of tick transmission of B. hermsii is influenced by the serotype infecting the ticks.